No description available.

Functional crystalline oxides with perovskite structure have a wide range of electrical properties such as ferroelectric, ferromagnetic, and superconductive, as well as unique properties that make them suited for a wide variety of applications including electro-optics, high-k dielectrics, and catalysis. Therefore, in order to realize the potential of perovskite oxides it is desirable to integrate them with semiconductors. Due to the high surface energy of oxides compared to that of semiconductors and the low number of oxides that are thermodynamically stable against SiO₂ formation, it has been extremely difficult to integrate epitaxial oxides with Si directly. However, in 1998, McKee and co-workers finally succeeded in depositing SrTiO₃ on Si directly using a Sr template via molecular beam epitaxy. This breakthrough opened the possibility of integrating the perovskite oxides with Si to realize potential device applications. In this dissertation, alkaline earth metal (Sr and Ba) templates on semiconductors, which enable epitaxial growth of complex oxides on semiconductors, are investigated using molecular beam epitaxy (MBE) for growth and in-situ X-ray/ultraviolet photoemission spectroscopy (XPS/UPS) for the electronic structure analysis. An epitaxial layer of SrTiO₃ on Si using such alkaline earth templates is used as a pseudo-substrate for the integration of perovskite oxides on Si. Through the use of post-deposition annealing as a function of oxygen pressure and annealing time, the strain relaxation behavior of epitaxial SrTiO₃ films grown on Si is also investigated to determine how the SiO₂ interlayer thickness affects the SrTiO₃ lattice constant. This ability to control strain relaxation can be used as a way to manipulate the properties of other perovskite oxides grown on SrTiO₃/Si. Additionally, SrTiO₃ can be made conductive by doping with La. Conductive SrTiO₃ can be used as a thermoelectric, a transparent conductive layer, and a quantum metal layer in a quantum metal field-effect transistor (QMFET). The structural, electrical, and optical properties of strained conductive La-doped SrTiO₃ are studied in order to understand the relation between elastic strain and electrical properties for electronic device applications. Oxide quantum well systems based on LaAlO₃/SrTiO₃ are also investigated using spectroscopic ellipsometry to understand how the quantum well layer structure affects the electronic structure. Such quantum well systems are good candidates for the monolithic integration of functional perovskites on semiconductors. Oxides quantum wells can be used in various device applications such as in quantum well cascade lasers, laser diodes and high performance transistors. As part of the growth optimization for high quality complex oxide heterostructures, the surface preparation of SrTiO₃ substrates using several different methods was also extensively studied using angle-resolved photoemission spectroscopy (ARPES). We found that acid-free water-based surface preparation is actually more effective at removing SrOx̳ crystallites and leaving the surface TiO₂-terminated compared to the more commonly used acid-based methods. / text

We report the possibility to control the tilting of nanoneedles and nanowires by using structured sapphire substrates. The advantage of the reported strategy is to obtain well oriented growth along a single direction tilted with respect to the surface normal, whereas the growth in other directions is suppressed. In our particular case, the nanostructures are tilted with respect to the surface normal by an angle of 58°. Moreover, we demonstrate that variation of the nanostructures shape from nanoneedles to cylindrical nanowires by using SiO2 layer is observed.

In this study, we have investigated the effect of substrate temperature, use of adhesive layer, deposition rate, annealing and substrate prebaking on the morphology of gold films deposited onto quartz surfaces. For the film growth, physical vapor deposition methods namely electron beam and thermal depositions have been used. Surface morphology of the films have been characterized with atomic force microscopy. Our aim was to confirm the general trends observed for these parameters in our evaporator system for a limited working range in order to produce gold films which are suitable to be used simultaneously for quartz crystal microbalance and helium atom diffraction measurements. At the end of this study, we confirmed the general trends regarding the effect of these parameters stated in literature except annealing process. We obtained a minimum 170 nm2 atomically flat surface with a roughness value smaller than 0.200 nm by thermal deposition method.

Graphene is a single sheet of graphite. While bulk graphite is semimetal, graphene is a zero bandgap semiconductor. Band structure calculations show graphene has a linear energy dispersion relation in the low energy region close to the Dirac points where the conduction band and the valence band touch. Carriers in graphene are described as massless Dirac fermions in contrast to massive carriers in normal metals and semiconductors that obey a parabolic energy dispersion relation. The uniqueness of graphene band structure indicates its peculiar electronic transport properties. In this thesis work, single- and multi-layer graphene films epitaxially grow on either the Si face or the C face of SiC substrates in a homemade induction vacuum chamber by thermal decomposition of SiC at high temperatures. The surface morphology and crystal structure of epitaxial graphene are studied with surface analysis tools. The transport properties of epitaxial graphene are studied by magnetotransport experiments. An epitaxial graphene film turns out to be a multilayered graphene because carriers in epitaxial graphene act as those in single layer graphene. Top gated and side gated epitaxial graphene field effect transistors (FETs) have also been successfully fabricated. These systematic studies unambiguously demonstrate the high quality of epitaxial graphene and the great potential of epitaxial graphene for electronic applications

Surface modification has wide ranging implications in lubrication, microelectromechanical systems (MEMS), colloidal systems and biological membranes. Surface modification plays an important role in stabilizing gold nanoparticles, which have applications in targeted drug delivery and catalysis. A variety of surface modification techniques are used for controlling corrosion and wettability, as well as used extensively to understand the nature of interactions between surfaces. This thesis is mainly focused on understanding the kinetics, film growth and surface modification by long chain molecules physisorbed on a surface. The time evolution of film growth and domain formation of octadecylamine on a mica surface is studied using ex-situ AFM and reflectance FTIR. A novel technique of interface creation is developed to measure the height of the adsorbed film. The results show three distinct regions of film growth mechanism. Region I, corresponds to thin film and the interface height is in the monolayer regime. The transient regime (II) consists of a sharp increase in the film thickness, from 1.5 nm to 25 nm within a time span of 180 s. In the final stage of film growth the film thickness is invariant with time, during which domain coarsening is observed. Domain evolution reveals a non-monotonic variation in the domain size as a function of adsorption time. A three stage mechanism is proposed to explain the domain evolution on the surface. In order to explain the observed film thickness variation, we have developed and tested various models to explain the thin to thick film transition observed in the AFM experiments. A model based on adsorption kinetics is solved to obtain the evolution of the adsorbed film. The model with a two-step adsorption isotherm quantitatively captures the thin to thick film transition observed in the AFM experiments. The statistical thermodynamics of adsorption of long chain molecules on a surface has been studied using a lattice model. The molecules are characterized by backbone chain, either lying parallel or perpendicular to the surface. A square lattice with nearest neighbour interactions and a mean field approximation are used to generate the adsorption isotherms for different molecules as a function of chain length. The molecules change their orientation from a surface parallel to an upright configuration with an increase in chemical potential. A similar transition (with time) in the molecular orientation has been observed in the AFM experiments. The transition between these two orientations is accompanied by an entropy maximum The last part of the thesis is concerned with carbon-carbon interactions. More specifically, we are interested in the interactions between graphite surfaces and their modification in the presence of a lubricant or base oil. Diamond like carbon (DLC) AFM tips and highly oriented pyrolitic graphite (HOPG) have been used for this study. Experiments were carried out by treating HOPG graphite in hexadecane oil at different temperatures. It is observed that pull-off forces on bare graphite are smaller when compared to the treated surface. The magnitude of the pull-off forces increases with the temperature of the hexadecane oil bath. Presence of charged patches responsible for the higher adhesion have been confirmed using surface potential microscopy. Results also confirm the presence of a thin liquid-like hexadecane film at room temperature.

abstract: The realization of Silicon based photonic devices will enable much faster data transmission than is possible today using the current electronics based devices. Group IV alloys germanium tin (GeSn) and silicon germanium tin (SiGeSn) have the potential to form an direct bandgap material and thus, they are promising candidates to develop a Si compatible light source and advance the field of silicon photonics. However, the growth of the alloys is challenging as it requires low temperature growth and proper strain management in the films during growth to prevent tin segregation. In order to satisfy these criteria, various research groups have developed novel chemical vapor deposition (CVD) reactors to deposit the films. While these reactors have been highly successful in depositing high crystal quality high Sn concentration films, they are generally expensive set-ups which utilize several turbomolecular/cryogenic pumps and/or load-lock systems. An more economical process than the state-of-the art to grow group IV materials will be highly valuable. Thus, the work presented in this dissertation was focused on deposition of group IV semiconductor thin films using simplified plasma enhanced CVD (PECVD) reactors. Two different in-house assembled PECVD reactor systems, namely Reactor No. 1 and 2, were utilized to deposit Ge, GeSn and SiGeSn thin films. PECVD technique was used as plasma assistance allows for potentially depositing the films at growth temperatures lower than those of conventional CVD. Germane (GeH4) and Digermane (Ge2H6) were used as the Ge precursor while Disilane (Si2H6) and tin chloride (SnCl4) were used as the precursors for Si and Sn respectively. The growth conditions such as growth temperature, precursor flow rates, precursor partial pressures, and chamber pressure were varied in a wide range to optimize the growth conditions for the films. Polycrystalline Ge films and SiGeSn films with an Sn content upto 8% were deposited using Reactor No. 1 and 2. Development of epitaxial Ge buffers and GeSn films was accomplished using a modified Reactor No. 2 at temperatures <400oC without the aid of ultra-high vacuum conditions or a high temperature substrate pre-deposition bake thereby leading to a low economic and thermal budget for the deposition process. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

UV and deep-UV selective photodiodes from visible-blind to solar-blind were realized based on a Si-doped (InxGa1–x)2O3 thin film with a monotonic lateral variation of 0.0035<x<0.83. Such layer was deposited by employing a continuous composition spread approach relying on the ablation of a single segmented target in pulsed-laser deposition. The photo response signal is provided from a metal-semiconductor-metal structure upon backside illumination. The absorption onset was tuned from 4.83 to 3.22 eV for increasing x. Higher responsivities were observed for photodiodes fabricated from indium-rich part of the sample, for which an internal gain mechanism could be identified. VC 2016 AIP Publishing LLC.

No description available.

This dissertation undertakes theoretical and computational research to characterize and understand in detail atomic configurations and electronic structural properties of surfaces and interfaces at the nano-scale, with particular emphasis on identifying the factors that control atomic-scale diffusion and transport properties. The overarching goal is to outline, with examples, a predictive modeling procedure of stable structures of novel materials that, on the one hand, facilitates a better understanding of experimental results, and on the other hand, provide guidelines for future experimental work. The results of this dissertation are useful in future miniaturization of electronic devices, predicting and engineering functional novel nanostructures. A variety of theoretical and computational tools with different degrees of accuracy is used to study problems in different time and length scales. Interactions between the atoms are derived using both ab-initio methods based on Density Functional Theory (DFT), as well as semiempirical approaches such as those embodied in the Embedded Atom Method (EAM), depending on the scale of the problem at hand. The energetics for a variety of surface phenomena (adsorption, desorption, diffusion, and reactions) are calculated using either DFT or EAM, as feasible. For simulating dynamic processes such as diffusion of adatoms on surfaces with dislocations the Molecular Dynamics (MD) method is applied. To calculate vibrational mode frequencies, the infinitesimal displacement method is employed. The combination of non-equilibrium Green’s function (NEGF) and DFT is used to calculate electronic transport properties of molecular devices as well as interfaces and junctions.